Agitators formed of a strip of material twisted into a helix and having a cross-section with two reverse or double inflections are known. When such an agitator is disposed in a tube, a fluid flowing therethrough impinges upon the concave faces of the helical agitator. Pressure losses are notably reduced within a tube without affecting the conditions of heat transfer. Such a prior art device is shown in the applicant's French Pat. No. 78 26 785 bearing Publication No. 2,436,959. In this known device, the cross-section of the helically twisted material has a central portion thicker than the inflected lateral portions which have an arcuate profile.
Continued research enabled the Applicant to discover other cross-sectional shapes enabling the fluid circulation conditions to be improved and the manufacture of the agitators to be facilitated still further.